I'm not a big fan of books that devote pages to listings of code and this book fortunately doesn't do that a lot. They also print a URL to download the code, right above the listing which is a nice feature that I haven't seen in many books before. While on the layout itself, the book is about 400 pages, it's black and white, not the most inspired layout but still clear.
I learned a lot about Prototype from this book, beyond the $, $F and $$ bits that have since shown up in other libraries, most notably jQuery. In fact, there's a lot functionality that's probably overlooked by casual users. That's the greatest strength of this book - it shows you a lot of things you probably didn't know either library was capable of. Personally, I've been using Prototype and script.aculo.us for about 4 months now in a variety of small projects so there was a lot of material here that I probably would have discovered eventually, but this book covered a lot of material that I wasn't aware of.